1. Field of the Invention
The present invention relates to a thermal transfer sheet comprising a substrate sheet, a colorant layer provided on one side of the substrate sheet, and a heat-resistant slip layer provided on the other side of the substrate sheet through a primer layer. More particularly, the present invention relates to a thermal transfer sheet which can surely prevent printing-derived cockling, fusing to a thermal head or the like caused by thermal head-derived thermal damage to a primer layer provided between a substrate sheet and a heat-resistant slip layer and, at the same time, can meet a demand for a reduction in thickness of the thermal transfer sheet and has a high level of suitability for high-speed printing.
2. Background Art
Thermal dye transfer sheets and heat-fusion thermal transfer sheets are known as thermal transfer sheets. In the thermal dye transfer sheets, a dye layer comprising a sublimable dye and a binder resin is provided on one side of a substrate sheet, for example, a plastic film such as a polyester film. On the other hand, in the heat-fusion thermal transfer sheets, the construction is the same as that of the thermal dye transfer sheets except that an ink layer formed of a colorant-containing heat-fusion composition is provided instead of the dye layer. In use, these thermal transfer sheets are heated imagewise from the backside thereof by means of a thermal head or the like to transfer the dye in the dye layer or the ink layer to an object and consequently to form an image on the object.
In recent years, there is a tendency toward an increase in a printing speed in thermal transfer recording. The conventional thermal transfer sheets, however, cannot cope with this tendency. Specifically, when printing is carried out using the conventional thermal transfer sheet by conventional heat energy, satisfactory sensitivity in transfer cannot be provided. Further, for prints having a thermally transferred image, higher image density and higher image sharpness have become required. The sensitivity in transfer should be improved for meeting this demand. Methods which have been proposed for improving the sensitivity in transfer include one in which the thickness of the substrate sheet is reduced and printing is carried out by conventional heat energy and one in which heat energy at the time of printing is increased to provide desired sensitivity in transfer. In these methods, however, thermal damage to the thermal transfer sheet is so large that problems such as printing-derived cockling, fusing of the thermal transfer sheet to a thermal head, and breaking of a ribbon disadvantageously occur.
Further, there is an ever-increasing need for a reduction in size of printers. However, the difficulty of realizing power saving involved in the conventional thermal transfer sheets has hitherto been an obstacle to the reduction in size of printers. In order to realize good sensitivity in transfer with lowered power consumption, a reduction in thickness of the substrate sheet has been attempted. This, however, has led to the same problems as described above, that is, printing-derived cockling, fusing of the thermal transfer sheet to a thermal head, breaking of a ribbon or other problems.
On the other hand, the incorporation of a resin having, at 120° C., an elasticity G′ of not less than 103 Pa and a viscosity G″ of not less than 104 Pa as a primer component in a primer layer for a heat-resistant slip layer has been proposed in Japanese Patent Laid-Open No. 1653/2001. This method, however, suffers from the following problem. Specifically, in the case of printing of an image in which white and black images are present together, the primer component behaves as follows. In the printing of a white part, heating conditions for printing are mild, and the viscosity and the elasticity are high, while, in the printing of a black part, heating is carried out and, thus, the viscoelasticity of the primer component is lowered. Due to this behavior, printing-derived cockling disadvantageously occurs at the boundary part between the white part and the black part. Therefore, this method cannot cope with a reduction in thickness of the substrate and an increase in heat energy without difficulties.